Digital recording systems typically include means for moving magnetic media past writing and reading heads. The reading heads in such systems commonly include one or more magnetoresistive (MR) transducers for reading the magnetic media. The error rate of a digital recording system is defined as the number of bits incorrectly detected divided by the total number of bits transferred. The system error rate is dependent upon the bias current that is passing through the magnetoresistive transducer. If the bias current is too low, the MR transducer will be underbiased and the resultant output signal will be distorted, and thus will have a high error rate. If the bias current is too high, the MR transducer will be overbiased and the resultant output signal will be distorted, and thus will also have a high error rate. Somewhere between the bias currents which yield underbiased and overbiased signals lies a range of bias currents where the MR transducer is optimally biased. Within this range of bias currents the error rate is minimized.
Typically, manufacturers of MR transducers determine their optimum bias current by conventional methods such as by measuring the second harmonic distortion of the output signal, or by comparing the asymmetry of the amplitudes of isolated pulses. Once this optimized bias current is determined it is used for all MR transducers of the same design.
However, because of manufacturing tolerances in the magnetic head relating to the height and resistivity of the MR transducer stripe, the amount of bias current required to minimize the error rate will vary from head to head. Furthermore, over time, magnetic media passing over the magnetic head will wear down the MR transducer stripe, which will affect its height. Changes in the height of the MR transducer stripe will in turn affect the resistance of the MR transducer and the amount of bias current required to minimize the system error rate.